The invention concerns an arrangement for and a method of characterising the polarisation properties of an optical system.
Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated by the illumination system is projected by the projection objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate.
It is known that, in high-resolution imaging systems such as for example the above-described microlithographic projection exposure apparatus, particularly with high numerical apertures, the influence of the imaging system on the polarisation state of the radiation which in operation passes through the imaging system can no longer be disregarded. That is to be attributed to changes in the imaging contrast due to polarisation-influencing effects (for example stress birefringence induced by holder components in the material of the optical components such as for example lenses or mirrors, polarisation-influencing effects of dielectric layers and so forth).
It is thus desirable to determine the polarisation properties of such imaging systems, in particular of high aperture, as reliably as possible, in order on the one hand to draw appropriate conclusions concerning the polarisation-dependent imaging quality, and on the other hand to be able to take possibly suitable measures for manipulation of the polarisation properties.
U.S. Pat. No. 7,286,245 B2 discloses inter alia a method of and an apparatus for determining the influence of the polarisation state of optical radiation by an optical imaging system, wherein a defined entrance polarisation state is afforded in an object plane of the imaging system and wherein the exit polarisation state of radiation issuing from the imaging system is measured in pupil-resolved relationship within a predeterminable pupil region of the imaging system. The imaging system can be for example a projection objective of a projection exposure apparatus designed for the wavelength range around 248 nm or 193 nm.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials. In that case the problem which arises in connection with the above-mentioned characterisation of the polarisation properties is that the implementation of a measurement structure like that described hereinbefore using reflective optical components can lead to considerable problems in regard to the required structural space, going as far as lack of viability.